Similar to "butter face" or broken down "but her face" as in everything is hot on a girl except her face...
This is when her face IS, in fact, hot also! BUT... her voice sucks.
It's high and whiny or it's low and gross, or it just all around sucks, doesn't match the hottness of the girl.

Basically put, simply said, the girl is very attractive, "but her voice" sucks..

Yeahyeah, hottie-hot-hot Evelyn got up in front of the class and I had never heard her talk before, turns out she is a butter voice...!!